maint/build ~ add rollup for ESM generation

- TypeScript fails to correctly generate an ESM module with correct line endings
  - ref: <microsoft/TypeScript#18442 (comment)>
  - ref: <microsoft/TypeScript#18442>
  - ref: <microsoft/TypeScript#37582>
  - ref: <microsoft/TypeScript#35148>

.rollup.config.js

@@ -0,0 +1,19 @@
+// ref: <https://devhints.io/rollup>
+// setup: `npm i rollup @rollup/plugin-typescript` or `npm i rollup rollup-plugin-typescript2` (for visible TS error output)
+
+// import typescript from '@rollup/plugin-typescript';
+import typescript from 'rollup-plugin-typescript2';
+
+export default [
+	// ES module build (replaces broken basic TypeScript compilation)
+	// * ref: <https://github.com/microsoft/TypeScript/issues/18442> , <https://github.com/alshdavid/rxjs/blob/main/rollup.config.js#L10>
+	// * ref: <https://github.com/microsoft/TypeScript/pull/35148>
+	// * ref: <https://github.com/microsoft/TypeScript/issues/37582>
+	{
+		preserveModules: true,
+		input: ['src/index.ts'],
+		external: ['path', 'os'],
+		output: [{ exports: 'auto', dir: 'build/esm', format: 'esm', entryFileNames: '[name].mjs' }],
+		plugins: [typescript({ tsconfig: './tsconfig/tsconfig.esm.json' })],
+	},
+];

package.json

@@ -57,6 +57,7 @@
 		"build:cjs": "shx rm -fr build/cjs && tsc -p tsconfig/tsconfig.cjs.json",
 		"build:cjs/esm": "shx rm -fr build/cjs/esm-wrapper && shx mkdir -p build/cjs && shx cp -r src/esm-wrapper build/cjs",
 		"## build:esm * [2020-12-22; rivy] TS compiles to ESMs are broken due to extension mishandling (use `rollup`)": "tsc -p tsconfig/tsconfig.esm.json",
+		"build:esm": "shx rm -fr build/esm && rollup -c .rollup.config.js",
 		"build:umd": "shx rm -fr build/umd && tsc -p tsconfig/tsconfig.umd.json",
 		"build:tests": "shx rm -fr build/cjs+tests && tsc -p tsconfig/tsconfig.tests.json",
 		"build:types": "shx rm -fr build/types && tsc -p tsconfig/tsconfig.types.json && run-s --silent _:update:dist.types",
@@ -177,6 +178,8 @@
 		"remark-footnotes": "^3.0.0",
 		"remark-preset-lint-recommended": "^5.0.0",
 		"rimraf": "^3.0.2",
+		"rollup": "^2.36.1",
+		"rollup-plugin-typescript2": "^0.29.0",
 		"shx": "^0.3.3",
 		"ts-node": "^9.0.0",
 		"tsd": "^0.13.1",